1. Field of the Invention
This invention relates to a rotating anode X-ray tube.
2. Description of the Related Art
In general, X-ray tube assemblies are used in medical diagnostic systems, industrial diagnostic systems, etc. An X-ray tube assembly comprises a rotating anode X-ray tube that emits X-rays, a stator coil, and a housing that contains the rotating anode X-ray tube and the stator coil. The rotating anode X-ray tube includes a fixed shaft, a rotor provided for rotation around the fixed shaft as an axis, an anode target disposed on an end portion of the rotor via a joint portion, a cathode arranged opposite to the anode target, a vacuum envelope that contains these elements, and a coolant that fills the vacuum envelope. A clearance between the fixed shaft and the rotor is filled with a liquid metal.
In an operating state of the X-ray tube assembly, the stator coil generates a magnetic field to be applied to the rotor, so that the rotor and the anode target rotate. Further, the cathode emits an electron beam to the anode target. Thereupon, the anode target radiates X-rays as it is struck by electrons.
During the operation of the X-ray tube assembly, the anode target is heated to high temperature by heat input to the anode target. Specifically, the anode target is heated to high temperature when it is irradiated with the electron beam. In particular, an electron impact surface (focus) that is struck by the electrons is heated to high temperature. Accordingly, the temperature of the electron impact surface must be lower than the melting temperature of the material of the anode target.
To meet this requirement, a technique for cooling the anode target has been developed. For example, a technique for cooling an anode target by using a liquid metal as a heat transfer fluid near an electron impact surface is disclosed in U.S. Pat. No. 5,541,975 and DE644719. Use of this technique enables high cooling of the anode target.
In the disclosed technique described above, however, a seal portion for the liquid metal is formed near the electron impact surface. Since heat generated from the electron impact surface is transmitted to the seal portion, the seal portion is inevitably heated to high temperature and deformed. Since a clearance between a rotor and a fixed shaft is deformed, it is difficult to maintain a clearance for the sealing performance of the seal portion to be fully displayed. In consequence, the X-ray tube may possibly be rendered defective by a leakage of the liquid metal.
Techniques for preventing the seal portion for the liquid metal from being heated to high temperature is disclosed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 63-13302, Jpn. Pat. Appln. KOKAI Publication No. 5-258691, and Jpn. Pat. Appln. KOKAI Publication No. 5-144395.